Method of operating a pneumatic brake booster

ABSTRACT

In a method of operating an electromechanically actuatable pneumatic brake booster, an electromagnet which actuates a sealing seat of its control valve is energized in an interval by a short current pulse of maximum current strength with a subsequent ramp-like current rise, and the current being furnished to the electromagnet is decreased when a fixed value of the pressure that prevails in a master brake cylinder connected downstream of the brake booster is reached. When the hydraulic pressure introduced into the master brake cylinder reaches a nominal value, the current being furnished to the electromagnet is supplied by a pressure controller which maintains the hydraulic pressure constant.

The present invention relates to a method of operating a pneumatic brakeforce booster which includes a booster housing having its interiorsubdivided by a movable wall into a first chamber (vacuum chamber) and asecond chamber (working chamber), and a control housing accommodating acontrol valve that controls a pneumatic pressure differential which actsupon the movable wall, the control valve comprising a first sealing seatthat is operable by an actuating rod and, when opened, permitsventilating the working chamber, a second sealing seat which, whenopened, permits a connection between the two chambers, and an elasticvalve member which interacts with the two sealing seats, wherein thefirst sealing seat or another sealing seat interacting with the valvemember is operable independently of the actuating rod to the effect ofventilating the working chamber by an electromagnet actuatable by anelectronic control unit, and wherein the hydraulic pressure isdetermined which is introduced into a master brake cylinder connecteddownstream of the brake force booster.

International patent application WO 93/24353 discloses a method of thistype. When the method known in the art is implemented, a memorizednominal braking pressure value is constantly associated with theinstantaneous actuating speed of a brake pedal actuating the brake forcebooster and compared with a measured actual braking pressure value. Thebrake force booster is driven in response to the comparison result. Adisadvantage of the prior art method is that the pressure values arepreset by evaluation of the brake pedal movement so that a mereindependent activation (which is carried out without operation of thebrake force booster by the driver) cannot be realized. Thus, the systemfor implementing the known method is inappropriate for use in someapplications, for example, as a hill holder, for precharging thehydraulic brake system, or for collision avoidance. A pneumatic brakeforce booster with electromagnetic auxiliary control is disclosed inGerman patent No. 44 05 092 wherein the first sealing seat is providedon a cylindrical sleeve which is connected to the armature of anelectromagnet arranged in the control housing. To permit operation ofthe prior art brake force booster by the driver, there is provision of avalve piston which is in a force-transmitting connection with theactuating rod. The armature is axially supported on the valve piston andmay thus be entrained by the valve piston upon operation by the driver.In the event of independent activation, or energization of theelectromagnet, the first sealing seat will lift from the valve member,thereby permitting the atmosphere to flow into the working chamber.

German patent application No. 42 38 333 discloses a vacuum brake forcebooster having a control valve which is includes a third sealing seatthat is operable by an electromagnet and replaces the second sealingseat in terms of effect in an independent activation. The control valveis operated by the electromagnet in opposition to the direction ofoperation by the driver so that the valve member is moved away from thefirst sealing seat on the valve piston by displacement of the thirdsealing seat.

Therefore, an object of the present invention is to disclose a method ofoperating pneumatic force boosters of the above mentioned generic typewhich permits a quick pressure increase to a desired value and obviatesthe need for operation of the brake force booster by the driver.Preferably, the desired pressure value is below the limit which ismaximally achievable by the independent activation.

According to the present invention, this object is achieved because theelectromagnet is actuated by way of an invariably predefined currentvariation, and after a fixed pressure value in the master brake cylinderis reached, the hydraulic pressure is controlled by variations of theelectric current being fed to the electromagnet. To specify the idea ofthe present invention,

a) the electromagnet is actuated by way of a current pulse of a maximumcurrent strength without influencing the control valve,

b) whereupon the current being supplied to the electromagnet isslope-like increased beyond the opening of the first or the furthersealing seat until the fixed pressure value in the master brake cylinderis reached,

c) subsequently, the current is reduced to a value where the pneumaticpressure that prevails in the booster housing remains constant and thehydraulic pressure in the master brake cylinder rises simultaneously,and d) the current value adjusted in step

c) is varied so that the pressure introduced into the master brakecylinder corresponds to a nominal value produced by the electroniccontrol unit.

The current value adjusted in step c) is preferably varied so that thepressure introduced into the master brake cylinder remains constant.

The present invention will be explained in detail in the followingdescription of an embodiment, making reference to the accompanyingdrawings.

In the drawings,

FIG. 1 is a greatly simplified view of a brake system design forimplementing the method of the present invention.

FIG. 2 is a longitudinal cross-sectional view, partly broken off, of anindependently activatable brake force booster in the inactive stand-byposition.

FIG. 3 is a diagram showing an independent activation of the brake forcebooster shown in FIG. 2, especially the time variations of the currentsupplied to the electromagnet of the brake force booster (FIG. 3a) andthe pressure increase in the master brake cylinder connected downstreamof the brake force booster (FIG. 3b).

The brake system for automotive vehicles shown in the embodiment of FIG.1 mainly comprises an actuating unit 1, an electronic control device 8and non-illustrated wheel brakes of an automotive vehicle. The actuatingunit 1, in turn, includes a pneumatic brake force booster, preferably avacuum brake force booster 2, which is operable both by an actuatingpedal 4 and independently of the actuating pedal 4. Connected downstreamof booster 2 is a master brake cylinder, preferably a tandem mastercylinder 3, having pressure chambers (not shown) which are connected tothe wheel brakes by way of hydraulic lines 9, 10. An actuating rod 5which is used to mechanically actuate a control valve 6 (shown onlyschematically) is coupled to the actuating pedal 4. Control valve 6controls the increase and decrease of a pneumatic differential pressurein the housing of the vacuum brake force booster 2. An electromagnet 7permits an (independent) activation of the control valve 6 independentlyof the actuating force which is introduced on the actuating rod 5 by thedriver of the vehicle. The electromagnet 7 is actuated by actuatingsignals of the electronic control device 8 by way of a current controlelement 45, the structure of which will be explained in the followingtext. A pressure sensor 11 connected to one of the pressure chambers ofthe tandem master cylinder 3 senses the hydraulic pressure introducedinto the tandem master cylinder 3.

The electronic control device 8 generally includes an electronic controlunit 12, a current control 13 and a pressure controller 14. Theelectronic control unit 12 to which the output signal p_(ist) of theabove mentioned pressure sensor 11 is sent, produces a nominal pressuresignal p_(soll) which is compared in a comparator circuit 43 with theactual value signal p_(ist) representative of the hydraulic pressurethat prevails in the master brake cylinder 3. The comparison result issent as an input quantity to the pressure controller 14. Further, thecontrol unit 12 produces control signals E_(s) as a function of theactual pressure value p_(ist). The control signals are taken intoaccount for deciding whether the electromagnet 7 shall be actuated bythe current signals I_(s) of the current control 13 or by the outputsignals I_(p) of the pressure controller 14. The mentioned change-overfunction is represented by the illustration of a selector switch 44.

FIG. 2 shows an embodiment of an independently activatable vacuum brakeforce booster which can be used for the mentioned pressure controlpurposes. The booster housing 20 (represented only schematically) of thebrake force booster is subdivided by an axially movable wall 22 into aworking chamber 23 and a vacuum chamber 24. The axially movable wall 22includes a diaphragm plate 28, deep drawn from sheet metal, and aflexible diaphragm 29 (not shown) abutting on the plate. The diaphragm,configured as a rolling diaphragm, provides a sealing between theoutside periphery of the diaphragm plate 28 and the booster housing 20.

The control valve 6, mentioned with respect to FIG. 1, is operable bythe actuating rod 5 and accommodated in a control housing 25, which issealed and guided in the booster housing 20 and carries the movable wall22. The control valve 6 is composed of a first sealing seat 15 providedon a valve piston 26 coupled to the actuating rod 5, a second sealingseat 16 provided in the control housing 25, a third sealing seat 30interposed radially between the two sealing seats 28, 29, and an annularvalve member 31 cooperating with the sealing seats 15, 16, 30. Valvemember 31 is guided in a guide part 32 sealed in control housing 25 andis biassed against the valve seats 15, 16, 30 by a valve spring 33 thatis supported on the guide part 32. The working chamber 3 is connectableto the vacuum chamber 4 through a channel 41 which extends laterally inthe control housing 25.

By way of a rubber-elastic reaction disc 36 abutting on the frontal end17 supported on the control housing 25 and a push rod 18 including ahead flange 21, the brake force is transmitted onto an actuating pistonof a non-illustrated master cylinder of the brake system. The masterbrake cylinder is arranged on the vacuum-side booster housing half notshown.

To connect the working chamber 3 to the atmosphere when the controlvalve 6 is actuated, a channel 27 which extends in a generally radialdirection is provided in the control housing 25. The return movement ofthe valve piston 26 at the end of a braking operation is limited by atransverse member 34 which, in the release position of the vacuum brakepower booster shown in the drawing, abuts on a stop 19 provided in thebooster housing 1.

To initiate an independent activation of the brake force booster shownin FIG. 2 irrespective of the actuating rod 5, the electromagnet 7mentioned with respect to the FIG. 1 embodiment is preferably arrangedin a housing 35 rigidly connected to the valve piston 26. The result isthat the electromagnet 7 is displaceable along with the valve piston 26in the control housing 25. The electromagnet 7 includes a coil 36accommodated within the housing 35 and an axially slidable cylindricalarmature 37. Armature 37 is partly guided in a closure member 38 closingthe housing 35. A force-transmitting sleeve 39 which carries theabove-mentioned third sealing seat 30 is supported on armature 37.Interposed between the valve piston 26 and the force-transmitting sleeve39 is a compression spring 40 which maintains the armature 37 in itsinitial position where the third sealing seat 30 is axially offset withrespect to the first sealing seat 15 provided on the valve piston 26.The closure member 38 guided in the control housing 25, by theintermediary of a transmission disc 42, bears against the abovementioned reaction disc 36 and permits transmitting the input forceintroduced on the actuating rod 5 to the reaction disc 36.

As can be seen in the diagram of FIG. 3, a short pulse of maximumcurrent strength with a subsequent slope-like current rise is applied tothe electromagnet 7 of the brake force booster shown in FIG. 2 at thecommencement of an independent activation (time T₀). Thus, theelectromagnet 7 is actuated so that the third sealing seat 30 in thefirst time interval T₀ -T₁ quickly moves towards the valve member 31until it abuts thereon and bridges the above-mentioned second sealingseat 16 in terms of effect. Upon further movement of the third sealingseat 30, seat 30 is initially urged into the material of the valvemember 31 in the actuating direction of the electromagnet 7, i.e. inopposition to the actuating direction of the brake force booster and,subsequently, displaces valve member 31 away from the first sealing seat15. The result is that a slot is produced at time T1 between the valvemember 31 and the first sealing seat 15 which enlarges with the currentrise and permits inflow of the atmosphere into the working chamber 23.In the above described operation where the second sealing seat 16 isreplaced by the third sealing seat 30 in terms of effect, pressure isincreased in the master brake cylinder 3 connected downstream of thebrake force booster 2 until a threshold value P_(schwell) is reached attime T2 when the current supplied to the electromagnet 7 is decreased toa constant value I_(h) and maintained on this value for a fixed period(until time T3). In this interval, the hydraulic pressure introduced inthe master brake cylinder 3 rises to the nominal value P_(soll)predefined in the electronic control unit 12 (FIG. 1). Preferably, thevalue I_(h) is chosen to be of such a quantity that the third sealingseat 30 moves until the valve member 31 closes the first sealing seat 15again, the third sealing seat 16, however, remains closed. The currentsent to the electromagnet 7 was supplied by the functional block`current control` 13 in the interval T0 to T3 (FIG. 1). At time T3 whenthe hydraulic pressure has reached its nominal value, the electroniccontrol unit 12 produces the change-over signal E_(s) which causes achange-over from the current control 13 to the pressure controller 14,if necessary, a P-controller. The control algorithm included in thepressure controller 14 adjusts the current supplied to the electromagnet7 so that the hydraulic pressure P_(hyd) in the master brake cylinder 3remains constant. As has already been mentioned, the hydraulic pressurewhich prevails in the master brake cylinder 3 is used as a referenceinput or nominal value, and the pressure controller 14 reads in thepressure value Psoll which prevails upon expiry of the interval T0 -T3.

We claim:
 1. A method of operating a pneumatic brake booster whichincludes a booster housing having its interior subdivided by a movablewall into a first chamber and a second chamber, and a control valve thatcontrols a pneumatic pressure differential which acts upon the movablewall, the control valve comprising a first sealing seat that is operableby an actuating rod and, when opened, permits ventilating the secondchamber, a second sealing seat which, when opened, permits a connectionbetween the two chambers, and a valve member which interacts with thetwo sealing seats, one of the sealing seats being operable independentlyof the actuating rod to the effect of ventilating the second chamber byan electromagnet actuatable by an electronic control unit,wherein thehydraulic pressure is determined which is introduced into a master brakecylinder connected downstream of the brake booster, and wherein theelectromagnet is actuated by way of an invariably predefined currentvariation, and, after a fixed pressure value in the master brakecylinder is reached, the hydraulic pressure is controlled by variationsof the electric current being fed to the electromagnet.
 2. The method asclaimed in claim 1, including the steps ofactuating the electromagnet byway of a current pulse of a maximum current strength without influencingthe control valve; thereupon slope-like increasing the current beingsupplied to the electromagnet beyond the opening of the first or thefurther sealing seat until the fixed pressure value in the master brakecylinder is reached; subsequently reducing the current to an adjustedvalue where the pneumatic pressure that prevails in the booster housingremains constant and where the hydraulic pressure in the master brakecylinder rises simultaneously; and varying the adjusted current value sothat the pressure introduced into the master brake cylinder correspondsto a nominal value produced by the electronic control unit.
 3. Themethod as claimed in claim 2, wherein the adjusted current value isvaried so that the pressure introduced into the master brake cylinderremains constant.